1. Field of the Invention
The present invention relates to a conductive copper paste composition containing copper metal powder which assures a satisfactory electrical conductivity. More particularly, the invention relates to a conductive copper paste composition which can be easily applied, for example by screen printing, to an insulation substrate to form a conductive circuit which, after thermal curing, is directly receptive to solders.
2. Description of the Prior Art
Having a resistivity of the order of 10.sup.-4 .OMEGA..multidot.cm, conductive silver paste has been widely used in the manufacture of printed circuits for electronic systems. However, silver powder is so expensive that it accounts for a significant proportion of production cost. Moreover, when a direct current voltage is applied to the silver paste conductive circuit in a high-humidity atmosphere, a migration of silver is induced to cause a short circuit. Therefore, development of a less expensive copper paste capable of replacing the silver paste has been demanded.
The known conductive copper paste consists of a copper powder and a thermosetting resin but when the film formed from this copper paste is cured by heating, the readily oxidizable copper combines with the oxygen in the air and the binder resin to form an oxide film on the surface of the copper particle, thus interfering considerably with its electrical conductivity. Moreover, with the progress of time the film formed from this copper paste loses its electric conductivity completely. To overcome this disadvantage, it has been proposed to use some additives to prevent oxidation of copper powder and thereby provide a copper paste with sustained conductivity (Japanese Patent Laid-open Specifications Kokai No. 61-31454).
However, these copper pastes have the problem that the resulting films are not directly receptive to solders. Thus, as the conductive circuits formed from the known copper pastes on insulation substrates cannot be directly soldered, it is necessary to subject the circuit film to activation treatment and electroless plating or to immersion copper electroplating using the film as the negative electrode before soldering. In such cases, the bond between the film and the copper plate must be positive and firm, and an equipment for disposal of plating effluents is also required. It was, therefore, clear that if a copper paste would permit direct soldering without need for electroless plating and/or electroplating, the process for formation of printed circuits would be drastically simplified and the consequent economic advantage would be remarkable.